Search for auroral belt E∥ fields with high‐velocity barium ion injections

Abstract

Four high‐velocity shaped charge Ba+ injections were conducted from two Black Brant‐10 rockets at collision‐free altitudes (770‐975 km) over northern Alaska (L = 7.4–10.6) in April 1984 under active auroral and magnetic disturbance (Kp 4+ and 5) conditions. The motions of the Ba+ “pencil” beams from these injections were accurately triangulated to altitudes ranging from 9000 to 14000 km from multistation image observations. Scanning photometer observations from five sites provided checks on the triangulations as well as auxiliary data, including observations that illustrate the lack of any anomalous ionization that might be expected from critical velocity effects. Well‐defined initial conditions and improved software for predicting the unperturbed, E∥ = 0, trajectories in the presence of convection, E⊥, fields permitted an accurate detection of changes in the motion which could be attributed to E∥ fields. Large (>1 keV) potential changes that might be anticipated from double‐layer or V‐, U‐, and S‐shaped potential structures were not encountered even though the Ba+ rays were clearly located on auroral arc flux tubes on at least several occasions and were at various times in close proximity to auroral flux tubes for many minutes. Abnormally intense E⊥ fields that might also indicate that the above potential structures were also not observed. Transient accelerations and/or decelerations involving magnetic field‐aligned energy changes ≤375 eV were, however, encountered by each of the seven principal Ba+ rays tracked to high altitudes. Acceleration events were only slightly more frequent than deceleration events. Interpretation, taking into account limits on the duration of the events and simultaneous auroral conditions, favors explanation in terms of propagating waves, soliton trains, or other pulse forms provided that the propagation is primarily field‐aligned. The similarity of energy and intermittent occurrence characteristics suggests that these E∥ perturbations are likely to be associated with suprathermal electron bursts, both upward and downward. One release, which uniquely took place in a region of intense (200 V/km) rotational shear in the E⊥ field, was followed by the appearance of a small population of suprafast Ba+ ions traveling well ahead of the main high‐velocity Ba+ rays. A second set of faint streaks, representing suprafast Ba+ ions from the low‐velocity debris cloud, simultaneously appeared above the debris rays. These suprafast ions are attributed to transverse energizations of small fractions of both the initial Ba+ jet and Ba+ debris immediately following the release. Transverse energizations for the highest velocity ions extended to 5879 eV with maximum flux near 952 eV. Tip motions for the fastest Ba+ debris ions were not obtained, but triangulation of one streak showed a maximum energization of 131 eV. Wave‐particle stochastic heating models may be relevant to explaining both the high energies and the dependence of the energy on initial velocities.